Most new hobbyists find alkalinity and calcium intimidating. To make it worse, a lot of LFS will not even take the time to explain this part of the process.
You are familiar with pH. Alkalinity is a measure of your aquariums ability to resist swings in pH. You should aim for an alkalinity reading of 8 - 12 dkh. The ability to test for alkalinity is extremely important, because it allows us to correct problems BEFORE they become a problem, so to speak.
This next part sounds complicated, but it isn't. Don't get caught up in the chemistry, just learn the how, and realize that there is a why. Have you ever asked yourself what makes ocean water different from water with salt added? Why can't you just add table salt to freshwater, and call it ocean water? The answer is this. There are many different salt ions in ocean water. These ions have a natural balance. This relationship is what makes water have a certain "feel". For example, have you ever noticed that water down south is very "soft" compared to water up north? This balance between these ions is very important to the fish. You have to keep this balance in check. Fortunately this is easily done, by testing for calcium.
Calcium is the primary buffering ion that makes up alkalinity. Remember, Alkalinity is a measure of your aquariums ability to resist swings in pH. Calcium is in a natural balance with other buffering ions, such as borate and magnesium. Again, don't worry about the chemistry. Just understand that testing for Calcium allows you to make changes before problems present themselves. You want to keep Calcium at 400 - 460 ppm.
So, on a weekly basis you will test for Alkalinity and Calcium. Follow this guide:
If Alkalinity is low and Calcium is low, add a buffer and add Calcium.
If Alkalinity is low and Calcium is high, add a buffer.
If Alkalinity is normal and calcium is low, add Calcium.
If Alkalinity is normal and calcium is normal, do nothing. However, test again mid week to determine how frequently you need to be taking the above steps to keep this balance in order.
If at any point you find that Calcium is beginning to drop at a rate faster than normal, this is a sign that you need a water change. Why? The other buffering ions are missing, causing calcium to be utilized at a faster rate than normal. The ions are out of balance, so to speak.
This all sounds complicated, but you just learned something critical. You learned WHY you are doing water changes, and how to tell if the water changes you are doing are sufficient. You also now have a method to reduce these water changes.
If you really understand what you read, then you now see how important a protein skimmer is. The skimmer removes organic acids. Acids remove buffers. The removal of buffers lowers alkalinity, causing pH swings, and causing an increased need for water changes to maintain the natural balance of the salt ion in ocean water.